There is an increasing need to protect electronic equipment against radiated and conducted electromagnetic interference. While shielding is the main means for protecting against radiated EMI, some equipment requires extensive and complex filtered circuits combined with shielding to meet standards required by industry and/or various government agencies. There is a increasing need, therefore, for means to limit the frequency content of information signals to prevent radiation and thereby reduce the need for elaborate shielding. This basic concept is known as "wave shaping" and it is accomplished by filtering a signal such that the shape of the fundamental signal is noticeably altered. By slowing down the rise time, the high frequency content of a signal is greatly reduced, which results in less radiation. The wave shaping is achieved by providing capacitance, inductance, and/or resistance, or combinations thereof in series with the signal.
Currently, wave shaping capability is provided by mounting respective leaded components on a circuit board. Such devices occupy a relatively substantial amount of surface area on the board. It is desirable, therefore, to have a means for accomplishing wave shaping that is compact, leadless, and lend themselves to being installed by automatic equipment. It is to be understood that in the context of this specification, the term "wave shaping" includes "filtering".
Filters are employed in electronic equipment, such as computers, for filtering out radio frequency interference ("RFI") or electromagnetic interference ("EMI") thereby precluding spurious operation of the electronic equipment due to the external influences exerted by other mechanical or electrical equipment in the environment of the electronic equipment. For high impedance, high frequency circuits pi-network filters are generally used. The term pi-filter, as known in the art, generally refers to a low pass filter. A low pass pi-network filter is comprised of a shunt capacitor at both ends and an inductor in series between them. The pi-network configuration may also be used to construct a high pass filter, which comprises an inductor at both ends and a capacitor in series between them.
The elimination of high frequency noise is necessary for the transmission of signals through the input/output devices of microcomputers, for example. Both individual pi-network filters and composite pi-network filter assemblies have been used in connectors. Many of these devices are formed as ceramic tubular sleeves having conductive layers plated on the inner and outer surfaces thereof. In most instances these filters then have a pin terminal secured to the inner surface and the filter pin assembly is then secured by its outer surface in an aperture of an infinite ground plane.
U.S. Pat. Nos. 4,553,114, 3,939,444, and 3,991,347 describe leaded devices that provide filtering on circuit boards. These devices generally are comprised of a filter sleeve mounted on a conductor, such that the ends of the conductor can be inserted into through holes of a circuit board, and a means for grounding the ground conductor on the filter sleeve. In the -347 patent, grounding is provided by directly soldering the sleeve to a ground conductor on the board. In the -114 and -444 patents, grounding is provided by a separate conductor member, thus requiring a third through hole in the circuit board for each filter device. While these devices serve to accomplish the desired filtering, they require the use of a relatively substantial amount of surface on the circuit board.
In addition the devices described and claimed in the aforesaid patents do not readily lend themselves to being installed by automatic equipment, such as robotic devices.